The invention relates to a defibrator for recycled materials such as waste paper.
In the known apparatus of the prior art, the material, such as waste paper, is defibrated by repeatedly falling against a hard surface, and this falling develops shearing forces which convert the paper into fibres without disintegrating the unwanted substances.
The operation is carried out in a rotating drum into which paper, moistened by the addition of water, is fed axially at one end. On its inner peripheral surface, the drum comprises longitudinal deflectors which lift the material when the drum rotates and lets it fall back just before it reaches the top of the drum, thus causing defibration. As the drum is slightly inclined relative to the horizontal, the material travels from the upstream to the downstream end of the drum, and during its retention time (18 to 20 minutes) in the drum, it is made to fall from the top of the drum on to the inner surface at the bottom 200 to 250 times.
The material then passes into a separator drum, also rotating, known as a sorting drum, having a perforated surface. Water is projected over the outer surface of the drum, thus forcing inwards any material lodged in the perforations. The fibres suspended in the water escape through the perforations in the bottom part of the rotating drum, whilst foreign matter (wires, plastics, etc.) are evacuated from the drum at the end of said drum.
These known machines have a variety of disadvantages:
the defibrating drum has to be considerable in size: in diameter, because the material has to fall from a sufficient height to produce the shearing forces needed for defibration; and in length, to ensure that any one portion of material is subjected to a sufficient number of falls in the course of its passage from the upstream to the downstream end of the defibrating drum; PA1 the drive torque to be applied to the drum is considerable, as the material is all contained in one side of the drum. This also results in an imbalance which fatigues the bearings and causes premature wear thereof; PA1 as the drum operates in only half its section, the space taken up by the drum is not used to full advantage. PA1 1. The material is distributed over almost all the defibrating drum and not just one side of it. The drum can therefore receive, per unit of length, almost double the amount of material treated in known apparatus. PA1 2. As has already been pointed out, the material coming into contact with the blades of the rotor and then with the inner wall of the drum is ground twice each time the drum rotates. PA1 3. Because of this, the shearing forces no longer depend on the diameter of the rotor. PA1 4. Consequently, for the same capacity, the drum may have a diameter slightly larger than half the diameter of a conventional drum. The length and the retention time can be reduced by about half.